95 Carina with no electrics

I hve had a flat battery for a few months, I know it needs replacing but these things cost quite a lot. I have been charging it up every now and then or using jumps leads when the battery went completely flat. The car has already started immediately when I use jump leads so there is no other mechanical problem.

However, the other day when I was in a bit of a hurry, I put the jump leads on the wrong way on my Toyota, now it won't even start using jump leads and all the electrics no longer work. What have I done?